1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use in a continuously variable transmission or the like.
2. Description of the Related Art
Heretofore, laminated rings for use in continuously variable transmissions or the like have been manufactured as follows: The ends of a thin sheet of maraging steel which is an ultra-high strength steel are welded to each other, producing a cylindrical drum, which is subjected to a first solution treatment. Then, the drum is cut into a ring having a predetermined width. After the ring is rolled, it is subjected to a second solution treatment. The ring is corrected to a predetermined circumferential length, and then aged and nitrided. A plurality of rings thus treated are laminated into a laminated ring.
The first solution treatment that is effected on the welded drum in order to uniformize the hardness that has been locally increased due to the heat applied when the thin sheet of maraging steel was welded. The first solution treatment allows the ring cut from the drum to be rolled with ease. Generally, the solution treatment is carried out by heating the maraging steel to a temperature equal to or higher than the recrystallization temperature of the maraging steel. The maraging steel contains aging precipitation strengthening elements of Ti, Al, Mo, etc. If these elements, particularly Ti, are oxidized, then a desired hardness may not be achieved by the subsequent aging process. In order to avoid oxidization of the aging precipitation strengthening elements, the solution treatment is performed in a vacuum furnace.
The rolled ring has a rolled structure in which metal crystals are crushed by the rolling process. If the rolled structure remained unchanged, then nitrogen would not easily penetrate the ring in the subsequent nitriding process, and the ring would not uniformly be nitrided. The second solution treatment is effected on the rolled ring in order to recover the original metal crystal grains prior to being deformed for facilitating the nitriding process.
The second solution treatment is generally carried out by heating the maraging steel to a temperature equal to or higher than the recrystallization temperature of the maraging steel, and the aging precipitation strengthening elements should not be oxidized in the second solution treatment. Since the vacuum furnace is expensive, the second solution treatment is performed in a heating furnace which contains a reducing atmosphere. The reducing atmosphere comprises, for example, a nitrogen atmosphere containing 1 to 10% of hydrogen. While the nitrogen atmosphere contains a small amount of oxygen, the oxygen is removed from the system by reacting with the hydrogen. Therefore, the concentration of oxygen in the nitrogen atmosphere is low enough to suppress oxidization of the aging precipitation strengthening elements. According to the above manufacturing process, since the second solution treatment is performed in the reducing atmosphere using the heating furnace, the number of vacuum furnaces that are used is reduced for cutting down on the cost of the vacuum furnaces.
However, an increase in the cost incurred in manufacturing the laminated ring is unavoidable because of different furnaces used in the first and second solution treatments.
The circumferential length of the ring which has been subjected to the second solution treatment is corrected as shown in FIG. 8 of the accompanying drawings. In FIG. 8, a ring 4 is trained under tension around a drive roller 21 and a driven roller 22. While the ring 4 is being rotated by the drive roller 21, a correction roller 23 disposed between the drive roller 21 and the driven roller 22 is moved in the direction indicated by the arrow. As a result, the ring 4 is loaded by the correction roller 23 in a direction perpendicular to the direction of travel between the drive roller 21 and the driven roller 22, and elongated to a predetermined length. In order to correct the circumferential length of the ring 4 appropriately, the ring 4 should preferably have an annular shape which is as close to a true circle as possible.
However, depending on the conditions of the second solution treatment, the ring 4 may suffer a thermal strain in the second solution treatment, and may be shaped to a complex irregular form, e.g., a substantially criss-cross shape as viewed in plan as shown in FIG. 10(a) of the accompanying drawings, or an extremely slender elliptical form as viewed in plan as shown in FIG. 10(b) of the accompanying drawings. If the ring 4 after the second solution treatment is of a shape as shown in FIG. 10(a) or 10(b), then when the ring 4 is trained around the drive roller 21 and the driven roller 22 for correcting the circumferential length thereof, the ring 4 may have portions that will not be brought into full contact with the rollers 21, 22. If the ring 4 thus shaped is corrected for its circumferential length, then the ring 4 may develop thickness irregularities as ring portions held in contact with the rollers 21, 22 and ring portions held out of contact with the rollers 21, 22 tend to have different thicknesses, and the ring 4 may have different circumferential lengths at opposite edges thereof.
It is therefore an object of the present invention to provide a method of manufacturing a laminated ring at a reduced cost by suppressing a thermal strain caused in a solution treatment after the ring has been rolled, thereby to correct the circumferential length of the ring appropriately with ease.
To achieve the above object, the inventors have studied conditions for a solution treatment to be effected on a welded cylindrical drum and a solution treatment to be effected on rolled rings. As a result, the inventors have found that if the conventional conditions for the solution treatment to be effected on rolled rings are made stricter, then such stricter conditions also applicable to the solution treatment to be effected on the welded cylindrical drum, and has completed the present invention based on the finding.
According to the present invention, there is provided a method of manufacturing a laminated ring, comprising the steps of preparing a cylindrical drum by welding opposite ends of a thin sheet of maraging steel, effecting a first solution treatment on the cylindrical drum, thereafter, severing the cylindrical drum into a plurality of rings each having a predetermined width, rolling the rings, effecting a second solution treatment on the rings which have been rolled, thereafter, correcting the circumferential length of each of the rings, thereafter, aging and nitriding the rings, and stacking the aged and nitrided rings into a laminated ring, the arrangement being such that the first solution treatment and the second solution treatment being effected using the same heating furnace in the same atmosphere in the same temperature range.
In the method according the present invention, a cylindrical drum is prepared by welding opposite ends of a thin sheet of maraging steel, and then placed into a heating furnace where a first solution treatment is effected on the cylindrical drum. Then, the cylindrical drum is severed into a plurality of rings each having a predetermined width, and the rings are rolled. The rolled rings are then placed into the heating furnace that has been used to effect the first solution treatment on the rings. In the heating furnace, a second solution treatment is effected on the rolled rings in the same atmosphere in the same temperature range as those of the first solution treatment, Thereafter, the circumferential length of each of the rings are corrected, and the rings are aged and nitrided. The aged and nitrided rings are stacked into a laminated ring.
By making stricter conditions for the second solution treatment to be effected on the rolled rings, the first solution treatment and the second solution treatment can be performed using the same heating furnace in the same atmosphere in the same temperature range. As a consequence, the laminated ring can be manufactured at a reduced cost because there is no need for separate furnaces to be used respectively for the first solution treatment and the second solution treatment.
Specifically, the stricter conditions for the second solution treatment to be effected on the rolled rings include limiting the range of dew points of a nitrogen atmosphere. The inventors have analyzed the relationship between the dew point of a nitrogen atmosphere used for the second solution treatment and the oxidization of aging precipitation strengthening elements in the first solution treatment. As a result, the inventors have found that when the dew point of the nitrogen atmosphere is in the range from xe2x88x927 to 0xc2x0 C., the aging precipitation strengthening elements, particularly, Ti, are less oxidizable. The inventors have also found that if the dew point of the nitrogen atmosphere is lower than xe2x88x927xc2x0 C., then Ti is selectively oxidized, and if the dew point of the nitrogen atmosphere is lower than xe2x88x9240xc2x0 C., then oxidization of Ti is suppressed again.
In the method according to the present invention, the first solution treatment and the second solution treatment are effected in a temperature range from the recrystallization temperature of the maraging steel to 850xc2x0 C. in either a nitrogen atmosphere that contains 1-10% of hydrogen and has a dew point ranging from xe2x88x927 to 0xc2x0 C., or a nitrogen atmosphere that contains 1-30% of hydrogen and has a dew point ranging from xe2x88x9270 to xe2x88x9240xc2x0 C.
The nitrogen atmosphere contains a trace of oxygen and also contains hydrogen in the above range. Therefore, the oxygen is removed by being combined with the hydrogen, so that the concentration of oxygen in the nitrogen atmosphere is lowered.
If the dew point of the nitrogen atmosphere were higher than 0xc2x0 C. in the first solution treatment, then the concentration of oxygen in the nitrogen atmosphere would be high, making it difficult to suppress the oxidization of Fe, which is the base material of the maraging steel, and also the oxidization of the aging precipitation strengthening elements of Ti, Al, Mo, etc.
If the dew point of the nitrogen atmosphere is in the range from xe2x88x927 to 0xc2x0 C., then the concentration of oxygen in the nitrogen atmosphere is lowered to such a level that Ti, Fe, and other aging precipitation strengthening elements of Mo, etc. are competitively oxidized. In this level of oxygen concentration, since a large amount of Fe existing as the base material is mainly oxidized, the oxidization of Ti is relatively suppressed.
If the dew point of the nitrogen atmosphere were in the range from xe2x88x927 to xe2x88x9240xc2x0 C., then the concentration of oxygen in the nitrogen atmosphere would be further lowered. Since Fe and Mo are reduced in this range, only Ti would selectively be oxidized.
If the dew point of the nitrogen atmosphere is in the range from xe2x88x9270 to xe2x88x9240xc2x0 C., then the concentration of oxygen in the nitrogen atmosphere is highly lowered, making it possible to suppress the oxidization of Ti again. The nitrogen atmosphere is normally produced by evaporating liquid nitrogen. Since the dew point of the nitrogen atmosphere immediately after the evaporation of liquid nitrogen is xe2x88x9270xc2x0 C., it is not practical to make the dew point lower than xe2x88x9270xc2x0 C.
The nitrogen atmosphere is identical to the reducing atmosphere used in the second solution treatment in the conventional method of manufacturing a laminated ring, except that the conditions for the dew point are stricter. Therefore, no problem arises from using the nitrogen atmosphere for the second solution treatment.
Therefore, both the first solution treatment and the second solution treatment can be performed using the heating furnace under the same conditions.
If the nitrogen atmosphere contained less than 1% of hydrogen, then the trace of oxygen contained therein would not be removed sufficiently effectively. Furthermore, inasmuch as hydrogen is more expensive than nitrogen, if the amount of hydrogen exceeded 10% of the nitrogen atmosphere when the dew point is in the range from xe2x88x927 to 0xc2x0 C., or if the amount of hydrogen exceeded 30% of the nitrogen atmosphere when the dew point is in the range from xe2x88x9270 to xe2x88x9240xc2x0 C., then the cost at which the laminated ring is manufactured would be increased.
If the temperature of the solution treatment were lower than the recrystallization temperature of the maraging steel, then the solution treatment itself would be difficult to perform, and if the temperature of the solution treatment exceeded 850xc2x0 C., then since recrystallized metal grains would become coarse, the notch toughness of the laminated ring would be lowered.
The dew point in the range from xe2x88x9270 to xe2x88x9240xc2x0 C. is preferable to the dew point in the range from xe2x88x927 to 0xc2x0 C. because it provides a wider dew point range for easy process control.
In the method according to the present invention, the second solution treatment is effected on the rings such that the rings after the second solution treatment in the atmosphere in the temperature range are of an annular shape.
Since the rings after the second solution treatment are of an annular shape, when each of the rings is trained around a drive roller and a driven roller for correcting its circumferential length, the ring can continuously be held in contact with the rollers. As a result, the rings are prevented from becoming irregular in wall thickness and also from having different circumferential lengths at opposite edges thereof, and the circumferential length of the rings can appropriately be corrected with ease.
If the temperature of the second solution treatment were lower than the recrystallization temperature of the maraging steel, then the solution treatment itself would be difficult to perform, and the rings would be deformed due to a thermal strain and shaped to a complex irregular form, e.g., a substantially criss-cross shape as shown in FIG. 10(a). If the temperature of the second solution treatment exceeded 850xc2x0 C., then the tensile strength of the rings would be lowered because the recrystallized metal crystal grains would be made coarse.
If the rings as suspended from a hook were heated to a temperature equal to or higher than the recrystallization temperature of the maraging steel, then the rings would be prevented from having the substantially criss-cross shape as shown in FIG. 10(a), but would possibly be shaped to an extremely slender elliptical form as viewed in plan as shown in FIG. 10(b). The rings would be shaped to an extremely slender elliptical form because they would suffer creep due to gravity depending on conditions for the solution treatment when heated while being suspended from the hook.
The second solution treatment is effected on the rings by placing a peripheral edge of each of the rings on a floor of the heating surface. With the peripheral edge of each ring being placed on the floor of the heating furnace, the ring is prevented from suffering creep due to gravity, but is easily be shaped to an annular form.
With the rings being placed directly on the floor of the heating furnace, the portions of the rings held against the floor of the heating furnace and the other portions of the rings tend to be heated differently. For uniform heating of the rings, the second solution treatment is preferably effected on the rings by placing the peripheral edge of each of the rings on a metal mesh mounted on the floor of the heating surface.
The heating surface comprises a loading zone for replacing the atmosphere with the nitrogen atmosphere and introducing the welded cylindrical drum or the rolled rings into the nitrogen atmosphere, a first zone for heating the cylindrical drum or the rings to the temperature range, a second zone for maintaining the cylindrical drum or the rings in the temperature range for a predetermined period of time, a third zone for cooling the cylindrical drum or the rings, and an unloading zone for replacing the nitrogen atmosphere with the atmosphere and unloading the cylindrical drum or the rings, the arrangement being such that the first solution treatment and the second solution treatment are effected by continuously supplying the cylindrical drum or the rings into the loading zone, delivering the cylindrical drum or the rings successively through the first zone, the second zone, and the third zone, and unloading the cylindrical drum or the rings from the unloading zone.
In the heating furnace thus constructed, the loading and unloading zones reliably isolate the atmosphere in the heating furnace from the external atmosphere, so that the atmosphere in the heating furnace is prevented from being disturbed when the drum or the rings are loaded into and unloaded from the heating furnace. Furthermore, since the welded drum or the rolled rings are continuously supplied to the heating furnace, and subjected to the solution treatment while being delivered successively through the first zone, the second zone, and the third zone, the solution treatment can continuously be effected on the drum or the rings.
The first solution treatment and the second solution treatment may be effected either separately for respective different periods of time or simultaneously for the same period of time.
The second solution treatment takes more time than the first solution treatment in order to recover the original metal crystal grains of the rings prior to being rolled. Therefore, if the first solution treatment and the second solution treatment are effected simultaneously for the same period of time, the period of time should preferably be equalized to the period of time required to effect the second solution treatment on the rings.
The rings that have been subjected to the second solution treatment are corrected for their circumferential length and then aged and nitrided. Since the oxidization of the aging precipitation strengthening elements of Ti, etc. is suppressed, the rings can be aged uniformly. Furthermore, inasmuch as the original metal crystal grains of the rings prior to being rolled are recovered in the second solution treatment, nitrogen can easily penetrate the rings, and hence the rings can easily be nitrided.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.